Mechanical and chemical-mechanical planarizing processes (collectively "CMP") are used in the manufacturing of electronic devices for forming a flat surface on semiconductor wafers, field emission displays and many other microelectronic device substrate assemblies. CMP processes generally remove material from a substrate assembly to create a highly planar surface at a precise elevation in the layers of material on the substrate assembly. FIG. 1 schematically illustrates an existing web-format planarizing machine 10 for planarizing a substrate 12. The planarizing machine 10 has a support table 14 with a top-panel 16 at a workstation where an operative portion (A) of a planarizing pad 40 is positioned. The top-panel 16 is generally a rigid plate to provide a flat, solid surface to which a particular section of the planarizing pad 40 may be secured during planarization.
The planarizing machine 10 also has a plurality of rollers to guide, position and hold the planarizing pad 40 over the top-panel 16. The rollers include a supply roller 20, idler rollers 21, guide rollers 22, and a take-up roller 23. The supply roller 20 carries an unused or pre-operative portion of the planarizing pad 40, and the take-up roller 23 carries a used or post-operative portion of the planarizing pad 40. Additionally, the left idler roller 21 and the upper guide roller 22 stretch the planarizing pad 40 over the top-panel 16 to hold the planarizing pad 40 stationary during operation. A motor (not shown) generally drives the take-up roller 23 to sequentially advance the planarizing pad 40 across the top-panel 16, and the motor can also drive the supply roller 20. Accordingly, clean pre-operative sections of the planarizing pad 40 may be quickly substituted for used sections to provide a consistent surface for planarizing and/or cleaning the substrate 12.
The web-format planarizing machine 10 also has a carrier assembly 30 that controls and protects the substrate 12 during planarization. The carrier assembly 30 generally has a substrate holder 32 to pick up, hold and release the substrate 12 at appropriate stages of the planarizing process. Several nozzles 33 attached to the substrate holder 32 dispense a planarizing solution 44 onto a planarizing surface 42 of the planarizing pad 40. The carrier assembly 30 also generally has a support gantry 34 carrying a drive assembly 35 that can translate along the gantry 34. The drive assembly 35 generally has an actuator 36, a drive shaft 37 coupled to the actuator 36, and an arm 38 projecting from the drive shaft 37. The arm 38 carries the substrate holder 32 via a terminal shaft 39 such that the drive assembly 35 orbits the substrate holder 32 about an axis B--B (arrow R.sub.1). The terminal shaft 39 may also rotate the substrate holder 32 about its central axis C--C (arrow R.sub.2).
The planarizing pad 40 and the planarizing solution 44 define a planarizing medium that mechanically and/or chemically-mechanically removes material from the surface of the substrate 12. The planarizing pad 40 used in the web-format planarizing machine 10 is typically a fixed-abrasive planarizing pad in which abrasive particles are fixedly bonded to a suspension material. In fixed-abrasive applications; the planarizing solution is a "clean solution" without abrasive particles. In other applications, the planarizing pad 40 may be a non-abrasive pad that is composed of a polymeric material (e.g., polyurethane) or other suitable materials. The planarizing solutions 44 used with the non-abrasive planarizing pads are typically CMP slurries with abrasive particles and chemicals.
To planarize the substrate 12 with the planarizing machine 10, the carrier assembly 30 presses the substrate 12 against the planarizing surface 42 of the planarizing pad 40 in the presence of the planarizing solution 44. The drive assembly 35 then 30 translates the substrate 12 across the planarizing surface 42 by orbiting the substrate holder 32 about the axis B--B and/or rotating the substrate holder 32 about the axis C--C. As a result, the abrasive particles and/or the chemicals in the planarizing medium remove material from the surface of the substrate 12.
The CMP processes should consistently and accurately produce a uniformly planar surface on the substrate to enable precise fabrication of circuits and photo-patterns. During the fabrication of transistors, contacts, interconnects and other features, many substrates develop large "step heights" that create highly topographic surfaces across the substrates. Such highly topographical surfaces can impair the accuracy of subsequent photolithographic procedures and other processes that are necessary for forming sub-micron features. For example, it is difficult to accurately focus photo patterns to within tolerances approaching 0.1 micron on topographic surfaces because sub-micron photolithographic equipment generally has a very limited depth of field. Thus, CMP processes are often used to transform a topographical surface into a highly uniform, planar surface at various stages of manufacturing the microelectronic devices.
In the highly competitive semiconductor industry, it is also desirable to maximize the throughput of CMP processing by producing a planar surface on a substrate as quickly as possible. The throughput of CMP processing is a function, at least in part, of the ability to accurately stop CMP processing at a desired endpoint. In a typical CMP process, the desired endpoint is reached when the surface of the substrate is planar and/or when enough material has been removed from the substrate to form discrete components on the substrate (e.g., shallow trench isolation areas, contacts, damascene lines, etc.). Accurately stopping CMP processing at a desired endpoint is important for maintaining a high throughput because the substrate assembly may need to be re-polished if it is "under-planarized," or components on the substrate may be destroyed if it is "over-polished." Thus, it is highly desirable to stop CMP processing at the desired endpoint.
In one conventional method for determining the endpoint of CMP processing, the planarizing period of a particular substrate is estimated using an estimated polishing rate based upon the polishing rate of identical substrates that were planarized under the same conditions. The estimated planarizing period for a particular substrate, however, may not be accurate because the polishing rate and other variables may change from one substrate to another. Thus, this method may not produce accurate results.
In another method for determining the endpoint of CMP processing, the substrate is removed from the pad and then a measuring device measures a change in thickness of the substrate. Removing the substrate from the pad, however, interrupts the planarizing process and may damage the substrate. Thus, this method generally reduces the throughput of CMP processing.
U.S. Pat. No. 5,433,651 issued to Lustig et al. ("Lustig") discloses an in-situ chemical-mechanical polishing machine for monitoring the polishing process during a planarizing cycle. The polishing machine has a rotatable polishing table including a window embedded in the table. A polishing pad is attached to the table, and the pad has an aperture aligned with the window embedded in the table. The window is positioned at a location over which the workpiece can pass for in-situ viewing of a polishing surface of the workpiece from beneath the polishing table. The planarizing machine also includes a device for measuring a reflectance signal representative of an in-situ reflectance of the polishing surface of the workpiece. Lustig discloses terminating a planarizing cycle at the interface between two layers based on the different reflectances of the materials. In many CMP applications, however, the desired endpoint is not at an interface between layers of materials. Thus, the system disclosed in Lustig may not provide accurate results in certain CMP applications.
Another endpointing system disclosed in U.S. Pat. No. 5,865,665 issued to Yueh ("Yueh") determines the end point in a CMP process by predicting the removal rate using a Kalman filtering algorithm based on input from a plurality of Linear Variable Displacement Transducers ("LVDT") attached to the carrier head. The process in Yueh uses measurements of the downforce to update and refine the prediction of the removal rate calculated by the Kalman filter. This downforce, however, varies across the substrate because the pressure exerted against the substrate is a combination of the force applied by the carrier head and the topography of both the pad surface and the substrate. Moreover, many CMP applications intentionally vary the downforce during the planarizing cycle across the entire substrate, or only in discrete areas of the substrate. The method disclosed in Yueh, therefore, may be difficult to apply in some CMP application because it uses the downforce as an output factor for operating the Kalman filter.